Electrothermal reduction of oxide ores or other oxide compounds



United States Patent 46, Int. Cl. C22b 5/12, 19/04 U.S. C]. 75-14 3 Claims ABSTRACT OF THE DISCLOSURE An enclosed reducing space is provided with a plurality of groups of electrodes arranged at different levels and sufficiently separated from one another to avoid short circuits between the different groups, the electrodes being spaced apart an increasing distance at progressively lower levels to vary the specific voltages impressed on a charge acting as a resistor therebetween. Joule effect heat is produced for liquefying completely the non-volatile materials resulting from the reduction operation, and the specific voltages applied to the charge decrease at progressively lower levels of the charge corresponding to the decrease in resistivity of the charge as the zinc oxide and carbon content of the charge are diminished so as to provide a more effective distribution of the electric current in the charge. Means is provided for increasing the refractoriness and resistivity of the lower part of the charge, and a combustive gas is introduced at the lower part of the charge to provide the heat for completing the metallurgical operation.

This invention relates to an improvement in the process of electrothermal reduction of zinc ores or other oxidized zinc compounds or mixed ores, for producing metals.

Zinc vapors are of course produced, and cadmium vapors and lead vapors are also produced since cadmium and lead are generally present in the zinc materials treated. Carbon monoxide together with the vapors of reduced volatile metals escape from teh reduction enclosure, while the residues remain inside the enclosure until they are liquefied. Such residues are formed by the gangue of the ore consisting essentially of CaO, SiO A1 0 MgO, B30 and non-volatile metals such as Fe, and Cu, together with some non-volatilized lead and sulfur which was originally present in the ore. There may be also some coke not consumed during the reduction operation. Such residues are molten, and the molten residues escape by gravity from the furnace enclosure.

It is known according to the main patent application Ser. No. 252,947, now Patent 3,244,511, to use electric energy as a source of heat with a view of supplying the heat required for the endothermal reactions taking place in a metallurgical oven, to use the charge of the oven as a resistor for the electric current, and to subject the nongaseous product obtained from the reduction operation to a temperature which is sufficient to liquefy them, the liquid products thus obtained being drained out of the space occupied by the charge to be reduced according as they are produced.

However, the use of the charge as a resistor has met with difficulties owing to the variations of electric resistivity of the charge resulting from the progressive exhaustion of the letter in metals. The lines of current have thus a tendency to concentrate in the zones of the oven which contain the portion of the charge which is in the most reduced state, whilst the zone of introduction of fresh charge, which is colder and has a higher resistivity,

3,451,805 Patented June 24, 1969 is less favoured in that respect. The term progressive exhaustion as used herein means that the amount of zinc oxide as well as carbon content in the charge within the furnace or reduction enclosure diminishes continuously during the reducing operation. Accordingly, the composition of the material contained in. the reduction enclosure changes from what it was when it entered the furnace to the composition corresponding to the residues, such latter composition being present at the bottom of the furnace. The term resistivity signifies specific resistance of the substance which is the electrical resistance per centimeter of length of the conductor. That current distribution does not correspond to the optimum utilization of the volume available in the oven. Moreover, certain portions of the surfaces of the electrodes which serve to bring in the current, allow the passage of more current than other portions and this entails a more rapid wear of the electrodes.

The specific electrical resistance of the charge is therefore variable and is a function of its changing composition and of the temperature which is reached at various points. The fresh charge which contains a large amount of ZnO and carbon and has not yet reached a very high temperature, has a high electrical resistance. At the bottom of the furnace, the charge spent in ZnO and C, is very hot and has a specific resistance which is much smaller than at the top. Indeed, the high temperature and the fluid state of the residues act to reduce appreciably the resistance of the slags. (See: Martin, A. E., and Derge, Gerhard. The Electrical Conductivity of Molten Blastfurnace Slags. In AIME, vol. 154, 1943, pp. 104-115. (TS 300 A35.))

Such high temperature is also produced by the fact that the oxide of zinc and carbon disappear, thus suppressing the endothermal effect and raising the temperature of the mixture instead.

The present invention has for its object to avoid the said drawbacks, thus ensuring a more effective distribution of the electric current inside the space occupied by the charge to be reduced. The present invention employs Joule effect heat. According to Joule law, the thermal energy evolved by a conductor having a resistance R and through which passes an electric current of intensity I, is given by the relation:

W=RI2 1 Ohms law gives:

I=E/R or E=IR where E=electromotive force, and R=resistance.

Therefore, a given value of E, a variation of R produces an inversely proportional variation I; if R is very small, I may have very high value.

The thermal energy produced by Joule effect increases when I increases, according to the above relation (1).

If for example E=20 v. and R: 109

l=2 0/l0=2 amperes and W=R1 =10 2 =40 watts.

If R=2 and E=20 -v., then [:10 amp. and W=20 l0 =00 watts centimeter of a conductor through which an electrical current of an intensity I passes.

Another characteristic feature of the invention is that the electric current is supplied by a plurality of groups of electrodes arranged at different levels and sufficiently separated from one another to avoid short-circuitings between superposed groups of electrodes. Short circuits may be produced between adjacent electrodes, when the latter are not subjected to the same voltage. This happens when the difference of voltage applied to a pair of electrodes is different from that applied to the next pair. The current may then pass direct from one electrode to the electrode situated immediately above or underneath, with a very high intensity, the latter producing the destruction of the electrodes. Therefore, there is a minimum distance to be maintained between the electrodes of two adjacent groups, and such distance according to the voltages applied to the said electrodes, may vary from 0.25 meter to 1.0 meter.

Simple and advantageous means for carrying the invention into effect whilst maintaining the voltage at the terminals of the various circuits substantially unchanged, consists in effecting the variation of the specific voltage by increasing in a downward direction the spacing between the surfaces of electrodes belonging to a given electric circuit, so as to lengthen the lines of current from the surface of one electrode to the surface of the opposed electrode.

It is also possible according to the invention to suppress in a substantially complete manner the specific voltage at the lower part of the zone occupied by the charge to be reduced and to supply to that part the heat required for completing the metallurgical operation, by a combustion obtained by introducing a combustive gas in the lower part of said zone. At the bottom of the furnace the resistivity of the liquid residues is very small. In order to allow an amount of heat sufficient to liquefy the residues, a very high intensity of current is required (since W=RI and this makes it difficult to provide a solution to the problem of feeding the lower electrodes with an electrical currentthe voltage is low (different from that of the upper electrodes), and the current intensity is very high. It is preferable to bring an additional amount of heat to that level of the furnace by the combustion of a gaseous fuel, for instance by placing injection nozzles along the side walls (the said nozzles being made for instance of carbide of silicon), and resting upon the bottom of the furnace, which bottom of the furnace is provided with openings through which liquid residues escape.

Among the substances the combustion of which may ensure the generation of the heat required, the following may be cited:

a reducing substance which remains available in the charge after the reduction of the latter;

certain metals of low volatility, for instance iron,

reduced; and

certain elements of the gangue of oxidized ore admitted into the oven, which during the reduction operation were brought to a lower degree of oxidation, for instance, in the state of sulphides.

Such an arrangement may prove itself to be useful more particularly when charges are treated of which the re duction residues are particularly good electrical conductors, because in that case an extremely low voltage and unusually high intensities of current would be required to produce the energy. This would be prejudicial from the point of view of transforming the current and moreover implies problems from the constructional point of view for the oven, due to the large electrode surfaces which are required.

Brief description of the drawing The drawing is a longitudinal section through a furnace employed for carrying out the process of the present invention.

already Description of the preferred embodiment Referring now to the drawing, the furnace includes heat-insulated walls 10 defining a closed chamber therewithin. The charge is received in a trough 12 having slots 14 formed therethrough, bridges 16 of refractory material being disposed over the slots to prevent non-fused materials from flowing out of the trough along with liquid residues. The trough is supported by blocks 18 formed of refractory material, and a channel 20 is slightly inclined toward one end of the furnace and conducts liquid which flows downwardly through the slots 14 to a pocket (not shown) outside of the furnace.

Conical passages 24 supported by the upper wall of the furnace are provided at their upper ends with gas tight closing devices 26. The lower ends of these conical passages open into the chamber defined within the furnace whereby the charge and other suitable materials may be introduced into the furnace.

A first pair of electrodes 30 are supported in spaced relationship to one another at the upper part of the furnace, these electrodes being supported in position by bars 32 connected in a suitable electric circuit. The electrodes and bars as employed in the present invention are both preferably formed of graphite. A second pair of electrodes 34 are supported opposite one another by bars 36 and are also connected in a suitable electric circuit. A third pair of electrodes 38 are supported opposite one another by bars 40, these electrodes also being connected in a suitable electric circuit.

It will be noted that the electrodes of each group or pair of electrodes are spaced apart an increasing distance at progressively lower levels within the furnace to thereby vary the specific voltage impressed on the charge disposed within the furnace and in engagement with the electrode. The groups or pairs of electrodes at different levels are also sufliciently separated from one another to avoid short circuits between adjacent groups or pairs of electrodes.

Pipes 44 extend through the walls of the furnace and into the chamber within the furnace, these pipes having nozzles 46 at the outer ends thereof for introducing a combustive gas into the lower part of the furnace.

Simple means for ensuring a better distribution of the electric current in the reduction zone consists in that the effect of the progressive exhaustion of the charge upon its resistivity is diminished by a preliminary introduction into the said charge of elements which, by their mixing with the non-gaseous residues of the reduction operation, increases the refractoriness and resistivity of the said residues. In this way, when the advanced exhaustion of the charge would tend to lower its resistivity to a too large an extent, the addition to the charge of those elements, lime, for instance, helps in maintaining its resistivity nearer to that of the non-exhausted charge. The addition of CaO to slags produces a certain increase of the melting point of the slags (see the following pertinent articles):

(1) Machin, J. S. and Hanna, D. L., Viscosity Studies of System CaO-MgO-Al O -SiO (I) 40 SiO In Journal of The American Ceramic Society. Vol. 28, No. 11, 1945, pp. 310-316. TP 785 A65.

(2) Machin, J. S., and Yee, Tin Boo. Viscosity Studies of Systems CaO-MgO-Al O -SiO (II) CaO-Al O -SiO In Journal of The American Ceramic Society. Vol. 31, No.7, 1948, pp. 200-204. TP 785 A65.

(3) Machin, J. S., Yee, Tin Boo, and Hanna, D. L., Viscosity Studies of System CaO-MgO-Al O -SiO (III) 35, 4S, and 50% SiO In Journal of The American Ceramic Society. Vol. 35, No. 12, December 1952, pp. 322- 324 TP 785 A65.

(4) Structure and Properties of Metallurgical Slags- Electrical Conductivity. In Erzmetall. Vol. 14, No. 9, 1961, pp. 44l450; and

(5) Martin, A. E., and Derge, Gerhard. The Electrical Conductivity of Molten Blast-Furnace Slags. In AIME. Vol.154, 1943,p. 104-115. TS 300A35.

What we claim is:

1. A process for the continuous production of metal vapors produced by reduction of oxidized zinc compounds by means of a carbonaceous reducing agent comprising the steps of providing apparatus including a plurality of groups of electrodes arranged at different levels and sufficiently separated from one another to avoid short-circuits between adjacent groups, the electrodes of each group of electrodes being spaced apart at an increasing distance at progressively lower levels to vary the specific voltage impressed on the charge, introducing a charge consisting essentially of an oxidized zinc compound and a carbonaceous reducing agent into an enclosed reducing space having said electrodes disposed therein, applying to said charge acting as a resistor electric current having an intensity sufficient to provide Joule eiTect heat for the reduction operation by which metal vapors are obtained and to liquefy completely the non-volatile materials resulting from the reduction operation, constantly avoiding an accumulation of liquid residues in the form of a liquid bath upon which charge would float by draining by gravity said liquefied residues from the yet unliquefied remainder of the charge, subjecting the charge acting as a resistor to specific voltages between said electrodes, said specific voltages decreasing at progressively lower levels of the charge in accordance with a reduction of the resistivity of the charge resulting from the progressive diminishing of the zinc oxide and carbon content, and lessening the etfect of the diminishing of the zinc oxide and carbon content of the charge upon the electrical resistivity of the charge by adding to the charge elements which by mixing with the non-gaseous residues from the reduction operation increase the refractoriness and the resistivity of the charge.

2. A process as defined in claim 1 wherein the specific voltage is substantially nulled at the lower part of the zone occupied by the charge to be reduced, and heat required for completing the metallurgical operation is generated by combustion obtained by introducing a combustive gas at the lower part of said zone.

3. A process as defined in claim 1 wherein the refractoriness of the charge is increased by the addition of lime to said charge.

References Cited UNITED STATES PATENTS 1,775,591 9/1930 Gaskill 13--23 2,754,196 7/1956 Wilkens et a1. 14 3,244,511 4/1966 Nicaise 75-87 1,800,500 4/1931 Bleecker 75-14 L. DEWAYNE RUTLEDGE, Primary Examiner. J. E. LEGRU, Assistant Examiner.

US. Cl. X.R. 1325; 75-87 

